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  Biosand Concrete Water Filter

What is it?

The Biosand filter is a slow sand water filter which is specifically designed for intermittent use. This allows the concrete filters to be built on a scale which is suitable for use by individual households. Prior to this improvement in design, slow sand filters were typically large-scale centralized community filters, reliant upon a continuous flow of water to ensure effective treatment. This design improvement to slow sand filter technology - a technology used effectively to treat water for more than 150 years - has allowed for a much broader spectrum of consumers to access it, at very low costs.

Slow sand filters have been proven to almost entirely remove the disease-causing organisms found in water. These organisms - known as pathogens - include bacteria, viruses, protozoa, cysts, helminths, and worms. The Biosand technological adaptation of slow sand filtration has proven as effective as traditional slow sand filters, in both laboratory and field tests. The technology has been introduced in more than 30 countries, by a diversity of organizations and people employing a diversity of means.

In conjunction with the introduction of the technology to communities, the filter has been tested by various government, research, and health institutions, as well as by non-governmental agencies.

What does it look like?

The filter is quite simply a concrete container, enclosing layers of sand and gravel.

Water is poured into the top of the filter as needed, where a diffuser plate placed above the sand bed dissipates the initial force of the water. Traveling slowly through the sand bed, the water then passes through several layers of gravel and collects in a pipe at the base of the filter. At this point, the water is propelled through plastic piping encased in the concrete exterior, and out of the filter, for the user to collect.

How does it work?

Pathogen Removal

As with all slow sand filters, the removal of pathogens occurs in the BioSand filter due to a combination of biological and mechanical processes. Organic material is trapped at - or very close to - the surface of the sand, forming a biological layer or 'schmutzdecke'. Over a period of one to three weeks, micro-organisms colonize this part of the filter, where organic food and oxygen derived from the water abounds. These micro-organisms consume bacteria and other pathogens found in the water, thereby providing highly effective water treatment. In addition to this process (known as predation), pathogens are removed as food scarcity and less than optimal temperatures causes their death and subsequent collection at the surface of the sand.

Viruses are adsorbed (become attached) to the sand grains. Once attached, they are metabolized by the cells or are inactivated by antiviral chemicals produced by the organisms in the filter.

Cysts and worms are removed from the water by becoming trapped in spaces which lie between the sand grains.

Between uses,a layer of water (5 cm deep) is maintained above the sand at all times. It is this design feature that distinguishes the Biosand filter from other slow sand filters and which allows for both small scale construction and for intermittent use.

How easy is it to use and maintain?

Operation

The following design features of the Biosand filter ensure its ease of use:

1. The water needs to be filtered only as required. As the safe storage of water in the household has been identified as a key risk area of disease prevention, this feature is extremely important.

2. The concrete plastic Biosand filter exists as a singular container, devoid of moving parts. Ensuring its successful operation lies well within the capabilities of any member of the household, including women and children. Operating the filter is very simple: remove the lid, pour a bucket of water into the filter, and immediately collect the treated water in a pail.

3. The concrete container is small, but extremely stable. Therefore, it can be placed anywhere in the home, in the location most convenient to the user.

4. The plastic piping is encased in concrete and is not easily damaged.

Maintenance

The biological layer typically takes one to two weeks to develop to maturity in a new filter. Removal efficiency and the subsequent effectiveness of the filter increases throughout this period.

Continued use of the filter causes the pore openings between the sand grains to become clogged with debris. As a result, the flow rate of water through the filter decreases. To clean the filter the surface of the sand must be agitated, thereby suspending captured material in the standing layer of water. The dirty water can then simply be removed using a small container. The process can be repeated as many times as necessary to regain the desired flow rate.

After cleaning, a re-establishment of the biological layer takes place, quickly returning removal efficiency to its previous level.

How much does it cost?

Capital

The cost of a concrete filter has varied from US$ 10 to US$ 30, depending upon the country in which it is being manufactured and the amount of paid/volunteer labour available.

The concrete filter utilizes the same technology as the commercial plastic filters produced by Davnor. However, using concrete to build the container costs less for several reasons:

• Cement is readily available in most developing countries.

• People are familiar with the construction techniques used.

• Household labour or volunteer labour can be utilized in the manufacturing process.

• The concrete container is heavy and durable. It does not need to be replaced as often as a plastic container.

• The plastic piping is located inside the filter. Consequently, it is less prone to damage than a plastic filter, which has piping on its exterior.

Operation

As consumables are not required for successful filter operation, the operating costs are negligible.

What difference can it make to the poor?

Biosand Filter Distribution Worldwide

Water is a resource vital to life, good health, and the development of human capacity.

Over the past few years, much money has been invested into the drilling of groundwater wells, as groundwater generally requires minimal treatment. However groundwater often contains contaminants which are difficult to remove and the development of affordable treatment processes - of which the Biosand technology is an excellent model - significantly expands source options, including the valuable ability to consider surface water sources, which can be more affordable and convenient to the consumer. Such technology also ensures greater reliability of water quality within each home. This is crucial, as water contamination in the aquifer, around the wellbore, or in the household is a frequent occurrence. Increasing the volume of clean water supplied by investing in low cost treatment of existing water sources, rather than in searching for additional ones, is both possible and advantageous.

The existence of a piped water supply to one's home is a convenience that every householder should be entitled to. However, it is not financially feasible to provide this level of service within the foreseeable future to the 1.2 billion people who currently do not have access to clean water. Therefore, it is vital to consider that individual household treatment systems can be just as effective as community-scale systems. Moreover, the particular needs of the poor are such that individual treatment systems are more likely to be socially and financially sustainable as well as more efficient and more easily replicated than community scale systems.

Of all the household treatment systems identified and evaluated by CAWST, the BioSand filter is the most affordable and efficient, as well as being easy for people of the community to use.

(c) 2002 Centre for Affordable Water and Sanitation Technology. This document may be freely reproduced while retaining this notice.

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